Alloy for service in petrochemical and hydrocarbon processing



United States atent US. Cl. 75-122 Claims ABSTRACT OF THE DISCLOSURENickel-chromium alloy containing especially controlled proportions ofcobalt, tungsten, niobium, aluminum, titanium and optional elements ischaracterized at high elevated temperatures, such as 800 C. to 1000 C.,by combination of properties including high strength and prolongedresistance to embrittlement, carburization and corrosion and isparticularly useful for reformer tubes and other fired pressure vessels.

The present invention relates to heat resistant alloys and, moreparticularly, to hot workable nickel-chromiumcobalt-iron alloys suitablefor long-time use at elevated temperatures.

A process of great importance in the petrochemical and hydrocarbonprocessing industries is the reforming of hydrocarbons by reaction withsteam in the presence of a catalyst to form a mixture of hydrogen andcarbon monoxide. This gas mixture is widely used under the name ofsynthesis gas for the synthesis of alcohol and also as a source ofhydrogen for ammonia production. The reforming reaction is carried outat high temperatures, usually in the range of 800 C. to 1000 C., underelevated pressure, which may be 400 pounds per square inch (p.s.i.), intubular reactors known as reformer tubes. These reactors are ofsubstantial size, being commonly up to 40 feet long, from 4 inches to 6inches in diameter and about 0.5 inch wall thickness, and are heatedexternally by the combustion of hydrocarbon fuels.

Providing a good material for reformer tubes presents a difiicultproblem. In order to be satisfactory, a reformer tube alloy must havegood tensile properties, in particular high tensile proof stress at theoperating temperature, and also have good resistance to creep understress. It must not embrittle on prolonged heating in the range of 800C. to 1000 C.; it must resist carburization; and it must also beresistant to corrosion by the combustion products of impure fuels. Owingto the large quantities of metal required for reformer tubes, it is alsoimportant that an alloy for such use be capable of being produced inrequired form at low expense; that is to say, it should have a low ratioof cost to strength.

Hitherto reformer tubes have frequently been made by centrifugal castingfrom chromium-20% nickel steels known by the Alloy Casting InstituteDesignation HK. Although such cast steel tubes have adequatestress-rupture strength for some purposes, they have some undesirablebrittle characteristics in the as-cast condition and sometimes becomeeven more brittle during elevated temperature service. They also tend tobe porous. A further disadvantage is that the centrifugal castingprocess produces a radial columnar grain structure and carburizationattack can proceed along the grain boundaries and lead to catas rophicfailure. There is accordingly a need for a new allow having an improvedcombination of properties required for reformer tube service and whichis hot-workable and weldable so that it can be formed into wroughtreformer tubes and other plant and apparatus that are subjected in useto similar conditions of stress, temperature and corrosive attack,including carburization.

Although many attempts have been made to overcome the foregoingdifficulties and other difficulties and disadvantages, none, as far aswe are aware, was entirely successful when carried into practicecommercially on an industrial scale.

It has now been discovered that a special combination of alloyingelements in new proportions enables providing a nickel-chromium alloyhaving improved characteristics of strength, toughness, resistance tocarburization and other characteristics needed for use in hydrocarbonand petrochemical processing at high elevated temperatures of the orderof 800 C. to 1000 C.

It is an object of the present invention to provide a newnickel-chromium alloy.

Another object of the present invention is to provide a hot workable,weldable nickelchromium-cobalt iron alloy having characteristicsparticularly well suited for use of the alloy under conditions ofexposure to carburizing atmospheres at elevated temperatures forextended periods of time.

The invention further provides a tubular reactor for use in reforminghydrocarbons under elevated temperature and pressure conditionsrequiring the reactor to be characterized by high elevated temperaturestrength, resistance to embrittlement and resistance to carburization.

Other objects and advantages will become apparent from the followingdescription.

Generally speaking, the present invention contemplates a hot workable,weldable nickel-chromium alloy containing carbon in an amount up to0.25%, e.g. 0.05% to 0.25%, about 19% to about 30% chromium, about 30%to about 50% nickel and about 2% to about 16% cobalt with the sum of thenickel and cobalt contents being at least 40%, 3% to 10% tungsten, 0.5%to 4% niobium, 0.15% to 0.6% aluminum, 0.15% to 0.6% titanium, up toabout 4% molybdenum, up to 0.75% silicon, up to about 1% manganese withthe balance, apart fro-m impurities, being essentially iron in amountsup to 37%. In addition, small amounts of other elements for purposessuch as deoxidation malleabilization, etc., e.g., up to 0.1% magnesium,can also be included in the alloy. All alloy composition percentages setforth herein are by weight.

It is important that each of the elements mentioned is present withinthe ranges specified. Carbon contributes to the tensile strength, and atleast 0.05% is preferably present, but at contents above 0.25% theimpact strength falls off. Advantageously, the carbon content does notexceed 0.1%. Chromium contributes to the resistance of the allOy tooxidation, and at least 19% is required. At chromium contents above 30%the alloys tend to embrittle on prolonged exposure to high temperatures.Beneficially, the chromium content does not exceed 25%.

Cobalt improves the stress-rupture life, impact-resistance andcorrosion-resistance of the alloy. At least 2% must be present, andadvantageously the cobalt content is from 2% to 10%. A total nickel andcobalt content of at least 40% is also necessary to avoid embrittlementon prolonged heating.

Tungsten and molybdenum make an important contribution to stress-rupturelife and tensile strength at elevated temperatures. At least 3% tungstenis required for this purpose, but if the tungsten content exceeds 10embrittlement occurs. Preferably, the tungsten content does not exceed6% when the total of the nickel and cobalt contents is less than 48%,and does not exceed 8% when the total nickel and cobalt content is 48%or more. Molybdenum supplements the effects of the tungsten at servicetemperatures up to 800 C. Above this temperature, the presence ofmolybdenum promotes excessive oxidation and for service at highertemperatures, e.g., 900 C. to 1000 C., the alloys are advantageouslymolybdenum-free.

The presence of small amounts of niobium in combination with thecontrolled amounts of other elements in the alloy has been found to bebeneficial to the resistance to carburization and green rot, and atleast 0.5% niobium must be present. Amounts of niobium greater than 4%lead to embrittlement and advantageously the niobium content does notexceed 2%.

As those skilled in the art will be aware, commercially availablesources Of niobium, e.g. nickel-niobium alloys, commonly contain smallproportions of tantalum, up to about one-tenth of the nominal niobiumcontent. Such amounts of tantalum present as an impurity in the alloysof the invention are not disadvantageous and are to be regarded as partof the niobium content.

At least 0.15% each of titanium and aluminium is required for adequatedeoxidation. If more than 0.6% of aluminium or titanium is present,carbides and intermetallic phases tend to precipitate during service,leading to weakening at the grain boundaries. While silicon can be addedas a deoxidant for the alloy, the amount of silicon present must notexceed 0.75% since larger amounts render the alloys susceptible toembrittlement and impair their weldability.

For obtaining an especially advantageous combination of characteristicsneeded in production and use of hydrocarbon reforming reactors, theinvention particularly provides a closely controlled alloy compositioncontaining carbon up to 0.1%, 19% to 22% chromium, 38.5% to 41.5%nickel, 7% to 9% cobalt, 4% to 6% tungsten, 0.5% to 1.5% niobium, 0.2%to 0.6% titanium, 0.2% to 0.4% aluminum, 0.6% to 0.9% manganese, up to065% silicon, up to 0.07% magnesium and the balance essentially iron.

For the purpose of giving those skilled in the art a betterunderstanding of the invention and/or a better appreciation of theadvantages of the invention, the following illustrative examples aregiven:

An alloy (alloy 1) which, in accordance with the invention, contained0.09% carbon, 20.55% chromium, 39.2% nickel, 8.0% cobalt, 4.33%tungsten, 0.89% niobium, 0.50% titanium, 0.38% aluminum, 0.34% silicon,0.83% manganese, 0.036% magnesium and the balance essentially iron wasprepared by air melting in a TABLE I Test temper- Stress Life Elongationture, C. {p.si.) (hours) (percent) TABLE II 0.1% 0.297. Ultimate Testproof proof tensile Elonga- Reduction Temp, stress stress stress tion ofarea C. (t.s.i.) (t.s.i.) (t.s.i.) (percent) (percent) t.s.i.=Long tons(2,240 pounds) per square inch.

TABLE III Impact strength at room temperature,

44.6 foot-pounds. 33.5 foot-pounds. 38.0 foot-pounds.

Heat treatment condition The characteristics of high impact strength andof resistance to embrittlement possessed by the new alloy, asillustrated by the test results in Table III, are a marked improvement,of greater order of magnitude, as compared with the as-cast impactstrength, by a similar test, of 1.5 footpounds for centrifugally-cast25% chromium20% nickel steel, which strength fell to 1.2 foot-poundsafter heating at 800 C. for 1000 hours.

In addition to possessing very good mechanical characteristics, asillustrated in the foregoing tables, the new alloy also has very goodresistance to carburization during exposure to reducing carbonaceousgases for prolonged periods of time, as illustrated hereinafter by testresults pertaining to alloy 2. The chemical composition of alloy 2,which is in accordance with the invention, is set forth in Table IV andcarburizing test results pertaining to alloy 2 are set forth in Table V.In order to further illustrate advantages of the invention, chemicalcompositions and test results pertaining to two other alloys (alloys Aand B), which do not contain niobium and are not in accordance with theinvention, are also included in Tables IV and V.

TABLE IV C Cr Ni Co W Mo Nb Al Ti Percent Percent Percent PercentPercent Percent Percent Percent Percent Fe 0. 08 20 36 8 6 1 0. 35 0. 37Ba]. 0. 08 20 36 8 6 1 0. 32 0. 42 Bal. 0. 08 20 36. 8 6 0. 36 0. 40Eat.

Ba1.=balance (including small amounts of silicon up to 0.4%, manganeseup to 0.8% and magnesium up to 0.05%)

Alloys having compositions set forth in Table IV were air-melted, castas ingots and then forged down to 0.75 inch diameter bar. Specimenstaken from bars of alloys 2, A and B were exposed to a mixture of carbondioxide with 40% by volume of carbon monoxide in a horizontal tubefurnace while being maintained at a temperature of 900 C. for prolongedperiods of 700 hours and 1000 hours. After being thus exposed to acarburizing atmosphere, the samples were descaled and the weight losswas determined. Weight losses which were undergone by the samples in theforegoing tests are set forth in Table V hereinafter. The test of alloyA was discontinued after 700 hours.

TABLE V Weight loss in milligrams per square centimeter After 700 hrs.After 1,000 hrs.

exposure exposure As illustrated in Table V, the alloy of the inventionhas superior resistance to carburization. Moreover, it is to beespecially noted that comparison of the weight losses after 700 hoursand 1000 hours for alloys 2 and B indicates that the rate of weight lossof alloy B was progressing much more rapidly than the weight loss ofalloy 2.

As pointed out hereinbefore, cobalt is an essential constituent of thealloys according to the invention, and is not equivalent to nickel. Theimprovement brought about by the replacement of nickel by cobalt may beillustrated by comparison of the properties of an alloy containing 40%nickel and 8% cobalt with those of an otherwise identical alloycontaining 48% nickel and no cobalt. The stress-rupture life of thelatter alloy at 900 C. was not more than one-fifth of that of the formeralloy and its impact strength in the annealed condition was less thanhalf of that of the former alloy.

The alloys can be readily welded and have good resistance to oxidationand corrosion. Apart from annealing to relieve stresses they do notrequire heat treatment to develop their strength properties.

The alloy of the persent invention is particularly applicable for theproduction of wrought products, including tubing, rod, plate, sheet,forgings, etc. Wrought products of the new alloy can be used for heatand corrosion resistant articles and structures, such as wrought, weldedor unwelded, reactor vessels, e.g., reformer tubes and supcrheater tubesfor boilers, and for other petrochemical and hydrocarbon processingapparatus which must endure exposure to heat, stress and corrosiveattack for extended periods of time. As indicated hereinbefore, specialembodiments of the alloy which are essentially devoid (free) ofmolybdenum, e.g., contain not more than 0.15 molybdenum, areadvantageous for use at temperatures exceed ing 800 C. and up to 1100C., e.g., 900 C. to 1100 C.

Reactor vessel made of the alloy provided herein have utilityparticularly in the petrochemical industry for the catalystic conversionof hydrocarbons to carbon monoxide and are generally useful in other gasplant apparatus, including fired pressure vessels.

We claim:

1. A hot workable alloy consisting essentially of carbon up to 0.25%,about 19% to about 30% chromium, about 30% to about 50% nickel and about2% to about 16% cobalt with the sum of the nickel and cobalt contentsbeing at least 40%, 3% to 10% tungsten, 0.5% to 2% niobium, 0.15% to0.6% aluminum, 0.15% to 0.6% titanium, up to about 4% molybdenum, up to0.75% silicon, up to about 1% manganese, up to about 0.1% magnesium withthe balance iron in an amount up to about 37%.

2. An alloy as set forth in claim 1 wherein the carbon content is atleast 0.05%.

3. An alloy as set forth in claim 1 wherein the cobalt content does notexceed 10%.

4. An alloy as set forth in claim 1 wherein the carbon content does notexceed 0.1%.

5. An alloy as set forth in claim 1 wherein the alloy is essentiallydevoid of molybdenum.

6. An alloy as set forth in claim 1 containing carbon up to 0.1%, 19% to22% chromium, 38.5% to 41.5% nickel, 7% to 9% cobalt, 4% to 6% tungsten,0.5% to 1.5% niobium, 0.2% to 0.6% titanium, 0.2% to 0.4% aluminum, 0.6%to 0.9% manganese, up to 0.65% silicon, up to 0.07% magnesium and thebalance iron.

7. An alloy as set forth in claim 6 wherein the carbon content is atleast 0.05%.

8. An alloy as set forth in claim 1 wherein the tungsten content doesnot exceed 8% and does not exceed 6% when the total of nickel pluscobalts is less than 48%.

9. A tubular gas reactor vessel subjected in use to stress and corrosiveattack at elevated temperatures of 800 C. and higher made of the alloyset forth in claim 1 and characterized by high strength and resistanceto impact and carburization during prolonged elevated-temperatureexposure for periods of at least 1000 hours at elevated temperatures ofat least 800 C.

10. An alloy as set forth in claim 1 containing 4% to 6% tungsten and0.5% to 1.5% niobium and essentially devoid of molybdenum.

References Cited UNITED STATES PATENTS 2,247,643 7/1941 Rohn et al. 1712,403,128 7/1946 Scott et al 75171 2,553,330 5/1951 Post et a1. 751222,763,543 9/1956 Wagner 75122 2,873,187 2/1959 Dyrkacz et al 751243,046,108 7/1962 Eiselstein 75171 CHARLES N. LOVELL, Primary ExaminerUS. Cl. X.R.

